Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on a disc. Modern disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The read/write transducer, e.g. a magneto resistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the head over the center of the desired track.
The heads are mounted via flexures at the ends of a plurality of actuator arms that project radially outward from the actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
Typically, such radial actuators employ a voice coil motor (VCM) to position the heads with respect to the disc surfaces. The actuator VCM includes a coil mounted on the end of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When controlled direct current (DC) is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces. The actuator thus allows the heads to move back and forth in an arcuate fashion between an inner radius and an outer radius of the discs.
The actuator arms are driven by a control signal fed to the VCM at the rear end of the actuator body. A servo system is used to sense the position of the actuator and control the movement of the head above the disc using servo signals read from a disc surface. The servo system thus relies on servo information stored on each disc. The signals from this information generally indicate the present position of the head with respect to the disc, i.e., the current track position. The servo system uses the sensed information to maintain head position or determine how to optimally move the head to a new position centered above a desired track. The servo system then delivers a control signal to the VCM to rotate the actuator to position the head over a desired new track or maintain the position over the desired current track.
Another parameter for accessing a desired track is the speed at which the disc rotates about the spindle axis. Disc rotation speed contributes to the time it takes the actuator arm to access a desired track, where a higher disc rotation speed allows for faster access times to the track and a slower disc rotation speed results in slower access time to the track. It is highly desirable in the disc drive art to have high disc rotation speeds and thus faster access times to the desired track.
An important aspect for accessing a desired track is the ability of the actuator arm, and hence the magneto resistive head, to maintain its proper positioning. For optimal performance, a head must maintain a centered position within the desired track. If the head becomes off-center within a track, even by a micro-inch, a tangible effect is noticed in the performance of the drive, i.e., lower overall reliability of the head to properly read or write-to a disc.
Critical to maintaining proper head positioning within a track is minimizing the effects of a phenomena called "disc flutter." Disc flutter or noise is generated by vibrations associated with the normal use of the disc drive system, where several contributing factors include: the rotation of the spindle, air turbulence around the periphery of the rotating disc or discs, the information storage disc thickness, the disc natural modes of frequency and frequencies and the clamping force exerted on the information storage disc. The combination of these factors can produce tangential waves that are at a maximum at the outer diameter or periphery of the information storage disc and at a minimum at the inner diameter of the disc. Additionally, as mentioned above, it is desirable to have higher disc rotation speeds for faster track access and thus faster spindle rotation. As such, the causes of disc flutter are increased as the rotation speed of the spindle increases. It is thus apparent that the effects of disc flutter are and will continue to be a major concern for the disc drive art.
Currently, a number of solutions have been implemented to minimize disc flutter, including: (1) incorporating air shrouds around the disc periphery to minimize air turbulence; (2) utilizing a more rigid spindle assembly to increase vibrational stability; and (3) utilizing higher modulus disc substrates and/or modulating the disc thickness to minimize the amplitude of any created noise. Although these solutions have had some benefit in reducing the effects of disc flutter, there is a need in the art to provide a more effective solution for minimizing and potentially eliminating the problem.